Cell surface features of B cells

The unique B cell surface feature is the antigen receptor, formed in part by membrane immunoglobulin. Receptor assemblies of newly formed mature B cells contain immunoglobulin M (IgM) and/or IgD. B cells appropriately stimulated by interaction with T helper cells and their interleukin products may switch to production of other Ig classes, and the B cell antigen receptors then contain IgG, IgA or IgE, but not IgM or IgD. The H chain of receptor immunoglobulin is anchored in the membrane, in association with proteins Ig-a (CD79a) and Ig-|3 (CD79b). The intracellular domains of Ig-a and Ig-(3 participate in generating a receptor-mediated signal within the cell, transmitted through activation of protein kinases. Lyn (Src family) and Syk (Syk/Zap70 family) are protein kinases that associate with the resting B cell receptor and, along with additional kinases, participate in B cell activation pathways.

Other B cell membrane proteins regulate the intensity of responses that follow antigen binding (Table 1). Cross-linking of a B cell-specific complement-binding assembly (proteins CD19, CD21, CD81 and Leu-13) to the antigen receptor dramatically lowers the antigen concentration required for B cell activation. CD45, a positive regulator (perhaps an initiator) of signaling, is a transmembrane protein with a cytoplasmic tail that has essential phosphatase activity. Other phosphatases (e.g. SHP-I) are attenuating, and some membrane molecules (e.g. CD22, FcyR) increase the effect of attenuating phosphatases.

Additional B cell surface molecules have diverse functions, serving to present processed antigen fragments and costimulatory signals to T cells, mediate adhesion between B and T cells, or as receptors for cytokines produced by T cells (Table 2). B cell activation and class switching can be stimulated through

Table 1 B cell receptors and associated membrane proteins

Receptor complex

Protein

Family

Function

B cell antigen receptor (BCR)

IgM, IgD, IgG, IgA or IgE

ig

Receptor-mediated signaling

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